1. Field of the Invention
In many imaging and detection systems for the imaging or detection of a source of electromagnetic radiation, it is desirable to provide an optical window which allows the transmission of the radiation of interest, but which provides electromagnetic interference (EMI) shielding and/or gives a low radar signature to help prevent detection of the optical aperture. Systems where such optical windows are provided include military platforms where it is desirable that the platform be hard to detect by radar, and in particular to target detection systems provided in such platforms, and to all forms of sensor or electrical systems which are sensitive to electro-magnetic interference.
2. Discussion of Prior Art
Known windows for use in such applications include those formed from a bulk semiconductor substrate, those coated with a continuous semiconductor or metallic coating, those formed as free-standing meshes and those formed as metallic meshes deposited on an optical substrate. Each of these known windows have their disadvantages.
A common form of optical window used, especially for EMI screening, is in the form of a fine regular linear metallic grid formed by conducting wires or tracks to form a regular square open structure on an optical substrate. In this case, the optical substrate is a multi-spectral optical material which allows transmission of a wide range of electromagnetic radiation wavelengths, including radio frequency energy.
Due to diffraction scatter and emission by the grid, there is the potential problem of increased noise due to the provision of the grid compared to a window comprising only an optical substrate. Especially where there is an intense source of radiation, the resulting image viewed through the optical window may include a two-dimensional array of spots centred around the actual point of incidence of the source of the radiation in the image plane due to the grid acting as a two-dimensional diffraction grating. In some circumstances, even when the actual source of radiation is outside the field of view of the optical equipment associated with the sensor, some false spots can be detected. Where the window of this type is used with a target detection system, this can lead to detection of a false target.
According to a first aspect of the present invention, a sensor system comprises:
a window including a mesh comprising an array of generally linear electrically conductive elements;
a sensor for detecting radiation of a predetermined wavelength or range of wavelengths passing through the window; and,
a means for determining the angle of incidence of the radiation at the sensor.
By determining the angle of incidence of the radiation at the sensor, it is possible to overcoming the problem of xe2x80x9cfalse spotsxe2x80x9d occurring in the detected image due to diffraction, and to clearly identify the location of the source. For example, in a target detection system, this ensures that the actual target is identified and located, and not an imaginary target corresponding to one of the false spots.
The sensor may be a charge coupled device. The detected image is preferably stored in memory for subsequent analysis.
Preferably the means for determining the location angle of incidence of the radiation includes a memory for storing a sample image representative of an expected image including the higher order diffraction effects due to the mesh, a comparator for comparing the actual image detected with the sample image overlayed sequentially at different locations of the actual image, and a detector for detecting the location at which the sample image corresponds most closely to the actual image, and determining the centre of the image as the centre of the sample image at that location.
The sample image stored in the memory may be a sample determined from an actual, known, sample source incident at a known location on the window of the sensor. Alternatively, a standardised image, such as a cross-pattern, or a simulated image determined mathematically can be used. In the case of a simulated image, this is preferable based on a Fourier transform of the grid at the desired frequency.
It is preferred that the apparatus further comprises a means to remove or compensate for the diffraction spots due to the higher order diffraction by the window at the predetermined wavelength or range of wavelengths. This results in the viewed or observed image comprising only the image incident on the window of the sensor without the higher order diffraction elements due solely to the provision of the grid.
The compensation means preferably alters the value of the pixels corresponding to the diffraction spots. Advantageously, this is achieved by selecting a group of pixels including the diffraction spot, and adjusting the value of the pixels forming the diffraction spot to correspond to the background. The pixels forming the diffraction spot are preferably determined by analysis of the value of the pixels in the group. This is advantageous as it compensates for any variation between the actual location of the diffraction spot and the expected location. Such variation may be due to the skew of the mesh or a variation in the frequency of the actual incident radiation compared to that for which the sample image was formed.
Preferably the sensor system includes a filter to prevent radiation with wavelengths outside the predetermined range passing through the window.
Preferably the mesh is formed on a transparent optical substrate. In this way, the substrate gives structure to the window allowing this to survive, particularly in military or industrial environments. This also allows for the window to seal an aperture in which it is used.
Where the mesh is formed on a transparent substrate, it is preferred that the substrate is selected as one having good transmissive properties at the wavelengths of radiation which it is desired to detect. Where electromagnetic radiation in the visible or longwave infra-red ranges is to be transmitted, it is preferred that the substrate is clear grade zinc sulphide or similar.
The grid may be formed on the outer surface of the optical substrate, or may be formed on the inner surface or be embedded within the optical substrate. Where the grid is formed on the substrate, an optically transmissive protective layer may be formed over the grid.
Although a discrete window may be provided, the mesh may be formed directly on the outer surface of the sensor.
A preferred application of the sensor is in a target detection system.
According to a further aspect of the present invention, a method for compensating for higher order diffraction resulting from a mesh on a window comprises the steps of:
comparing the detected image with a sample image located sequentially at different locations on the image; and,
determining the location of the sample image at which the actual image most closely corresponds to the sample image, and determining the point of incidence of the radiation forming the actual image as the centre of the sample image at that location.
With the method according to this aspect of the invention, it is possible to locate the point of incidence of the radiation on the window, and to avoid the problems associated with the detection of false spots.
With the method according to the invention, the angular location at which the detected radiation is incident on the sensor is determined. This is achieved by comparing the actual image detected, which includes the higher order diffraction effects, with a search or sample image applied sequentially at different locations. The sample image corresponds to an expected pattern or image from a point source incident at a particular location on a particular window. The sample image may be generated from a real source of known location and properties incident on the window or sensor. Alternatively, the image may be a generated image, such as a standard image such as a cross pattern which has been found to approximate the diffraction effects, or determined mathematically, for example using Fourier transforms.
The determination of the location of the sample image which most closely matches the real image is preferably achieved by comparing the value of individual pixels of the real and sample images, summing the results of these comparisons, and determining the location of the sample image giving the minimum difference between the two images. Whilst all pixels of the images can be compared, it is preferred that only a limited number of pixels are compared. In particular, it is preferred that only those pixels with a value above a certain threshold are compared, namely pixels corresponding to a certain level of brightness. By considering only a restricted number of pixels, the comparison process is quicker compared to the comparison of all pixels. Further, as only the brighter pixels correspond to the image which is to be detected, the comparison of only these pixels gives a more accurate result. The threshold above which pixels are considered may be a predetermined threshold, or may be based on an average of the value of the pixels of the image.
Preferably, the method comprises the further step of determining and removing the higher order diffractions spots of the actual image based on the expected effects for radiation incident at the determined location. In particular, once the location of the radiation on the sensor is known, and knowing the wavelengths of the radiation based on the known response of the system, the expected location of the diffraction spots can be determined. The pixels corresponding to the expected locations of the diffractions spots can then be adjusted to remove the spots, for example by making these correspond to a background level.
It is preferred that the spots are removed by taking a group of pixels including those which are expected to contain the diffraction spots, identify those pixels within the group which correspond to the diffraction spots, and adjusting the value of those spots. By taking a group of pixels including those expected to correspond to the diffraction spot, any slight variation between the actual location of the diffraction spot and the expected location, for example due to skew of the grid or due to variation in the wavelength of the radiation, can be compensated for.
It is preferred that the pixels in the group of pixels which correspond to the diffraction spot are determined by determining the frequency or number of pixels within the group of each value. In this case, it is expected that the determination will show two maxima, one corresponding to the background level and one to the level for the diffraction spots, separated by a minima. In this case, all pixels having a value greater than that at the minima can be determined to correspond to the diffraction spot. These pixels are preferably adjusted to correspond to the background level. This is preferably achieved by adjusting the value of the pixel to the average of the value of the surrounding pixels which are determined not to form part of the diffraction spot.